Process of making aluminum bonded stainless steel article



United States Patent 3,465,423 PROCESS OF MAKING ALUMINUM BONDEDSTAINLESS STEEL ARTICLE Allan Kiesler, Schenectady, William G. Moifatt,Ballston Lake, and James L. Walker, Schenectady, N.Y., assignors toGeneral Electric Company, a corporation of New York No Drawing. FiledOct. 14, 1965, Ser. No. 496,181

Int. Cl. B23k 31/02 U.S. Cl. 29-527.? 2 Claims ABSTRACT OF THEDISCLOSURE An aluminum coating is metallurgically bonded directly to astainless steel article by first cleaning the article of adherent oxideand then providing a protective coating of lead-tin solder on it; andafter as long an interval as desired, replacing the solidified solderfrom the article with aluminum by contacting the solder coating withmolten aluminum.

This invention relates to the bonding of aluminum or aluminum alloys tosteel, particularly to stainless steel.

Heretofore, there have been many processes for the bonding of aluminumto ferrous-base alloys. The Pershing Patent No. 2,634,469 discloses amethod for bonding aluminum to ferrous alloys by first coating theferrous alloy with a zinc-base alloy containing tin and aluminum. Thecoated base is then promptly placed in a mold or die and before the partcools below the point where the coating is mushy, molten aluminum oraluminum-base alloy is cast therearound. This process it notsatisfactory, however, because it requires handling of the coatedarticle while the coating alloy is still fluid.

The Zwicker Patent No. 2,849,790 discloses a process for the bonding ofaluminum to ferrous-base alloys by precoating the ferrous base with analloy containing tin and/or zinc and/or cadmium, together with certainrare earth elements. The light metal is then cast on the interface, andis united to the iron surface by a bonding layer consisting essentiallyof tin and zinc and the necessary rare earth metals.

The'bonding of aluminum to a ferrous metal is one problem of the art. Afurther and more difficult problem of the art is the bonding of aluminumto a stainless steel body. For example, Jepson Patent No. 3,096,566states at column 1, line 69: Unfortunately there has not been availableany simple method of bonding aluminum and stainless steel, so that thebonded structure will withstand the high temperatures experienced inelectric irons. This problem is attempted to be avoided by utilizationof a mechanical attachment of the aluminum to the stainless steel. Thestainless steel being a heat resistant and chemically inactive material,it is seen why it is so difficult to cast the aluminum onto thestainless steel.

It is an object of the present invention to provide a process forcoating stainless steel with aluminum.

It is a further object of the present invention to provide a process forcoating stainless steel with aluminum, wherein little or no intermediatemetal is used to bond the aluminum and the steel.

It is a still further object of the present invention to provide aprocess for the bonding of stainless steel and aluminum, so that it isnot necessary to transfer stainless steel with a molten coating of ametal thereon to a mold.

It has long been desired to bond stainless steel to aluminum. Theseresults have never been achieved because the aluminum will not adhere tostainless steel. The adherence of the aluminum in this process will bemaintained even under conditions where there is a large "Ice temperaturedifferential between the parts and/0r differential mechanical stressesapplied to the two materials. It has been found that the cause of thefailure to direct bonding between stainless steel and aluminum is a thincoating on the stainless steel of a mixed oxide of chromium and iron.The oxides of iron are usually reduced with molten aluminum. It isbelieved that the aluminum does not reduce the oxide of chromium. Thepresent process comprises the removal of any coating on the stainlesssteel and the protection of this surface by applying thereto a thinmetal coating. This coated article, after cooling, is then placed in amold and the aluminum cast thereon.

The casting step need not immediately follow the application of theprotective metal coating. The products provided with the protection maybe stored and have the aluminum cast around them at a later time.

It is desirable to clean the stainless steel surface before the metalcoating is applied and particularly to eliminate oxide skin, that is,the layer of lower oxides of iron and chromium which adversely affectsthe application and adherence of either the coating metal or thealuminum. To accomplish this end, an aqueous salt solution is preferred.An aqueous solution of zinc chloride has proven satisfactory. A solutionmay be applied to the surface of the stainless steel articles, and thenthey may be dried in a furnace at a temperature of approximately C. Thesurface thus treated must be quickly contacted with a molten metalcoating wtihout a surface rinsing step. The dried solution givesprotection to the cleaned stainless steel while the surface is beingcoated with the protective metal coating.

The metal coating need not form a serration bond with the stainlesssteel, but it must wet the steel and cover the same to provent thereformation of the iron and chromium oxide coating. These articles, whencoated with a lead/tin solder melting below 600 C., may be stored untilsuch a time as it is convenient to cast the aluminum on to the protectedsurface. The solder coating, it is believed, flows freely from theinterface surface, and allows the aluminum to form a metallurgical bonddirectly with the stainless steel, there being little or none of thesolder entering into the bond between the aluminum and the stainlesssteel.

As an example of this process, a stainless steel stamping of thefollowing composition, chromium 18 percent, nickel 8 percent, balanceiron, such as is used in the manufacture of the heating elements of acoffee percolator or electric steam iron, is cleaned with a zincchloride solution, dried and coated with a lead/tin solder containing 60percent lead, 40 percent tin. Also a 50 percent lead, 50 percent tinsolder has been found to give satisfactory results. The treated articleis then stored. This article is, at a later date, placed in a diecasting machine and molten aluminum is cast against this surface. Thealuminum adheres to the stainless steel and forms a metallurgical bondtherewith. The aluminum alloy used in this casting process contains 9 to10 percent silicon, 0.4 to 0.6

percent magnesium, the balance being aluminum.

The advantages of the present process are apparent when compared to theprocesses in which a ferrous base is coated with a metal interface, and,while this coating is still molten, has aluminum cast thereon. Thearticles of the present process may be prepared at the stamping mill,cleaned with a zinc chloride solution and coated with a protectivesolder. These can then be shipped or stored, awaiting the casting ofaluminum. The type of solder required in this process is simply suchthat it will wet the surface of the stainless steel, and protect it andprevent the formation of an oxide coating thereon. It is not necessarythat this solder form a serration bond with the stainless steel.

When reference is made to aluminum alloy in the claims, it iscontemplated that commercially pure aluminum of commerce is included inthe phrase.

Although the present invention has been described in connection withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A process of providing a bond directly between a stainless steel baseand aluminum cast thereon, comprising:

(a) contacting said stainless steel with a cleaning solution to removeadherent oxide,

(b) providing on the resulting clean oxide-free surface a protectivecoating of a solder having a meltingpoint temperature below 600 C.,

(c) cooling the resulting coated stainless steel article,

and

(d) subsequently removing the resulting solidified solder coating fromthe steel article and simultaneously casting aluminum or an aluminumalloy on said coated surface by bringing the solder-coated portion ofthe said article into contact with molten aluminum or aluminum alloy.

2. The method of claim 1 in which the solder is a leadtin solder and inwhich the molten aluminum or aluminum alloy is brought into contact withthe solder coating on the stainless steel article in a die-castingoperation.

References Cited UNITED STATES PATENTS 2,544,670 3/1951 Grange et a1164102 X 2,881,491 4/1959 Jominy et a1. 164-102 3,099,869 8/1963 Piper16475 X 3,165,983 1/1965 Thomas l6495 X 3,186,045 6/1965 Lagostina 16475PAUL M. COHEN, Primary Examiner US. Cl. X.R.

